Satellite based telecommunications systems have been proposed to provide cellular communications links between user terminals (mobile and fixed) and earth stations. The earth stations, in turn, connect the user terminals with remote originating/destination callers through public land mobile networks (PLMN), public switching telephone networks, other earth stations and satellites, and the like. Each user terminal communicates with an assigned earth station along corresponding forward and return links which are supported by a satellite which has the user terminal and earth station in its field of view.
Each satellite includes at least one antenna which defines its earth coverage region or footprint. The satellite antenna(s) divide the coverage region into multiple beam spots. Each beam spot is assigned at least one frequency subband along which communications signals travel in the forward and return directions between user terminals and earth stations. Each subband may support communications from a plurality of user terminals. The user terminals are assigned unique transmission channels or "circuits" within an associated subband. A channel or "circuit" represents a unique path along which the corresponding user terminal transmits and receives RF signals containing discrete frames or packets of communications data and/or command information. A channel or circuit may be defined in a variety of ways, depending upon the system's coding technique such as time division multiple access (TDMA), frequency division multiple access (FDMA) code division multiple access (CDMA), or any combination thereof.
The transmitters in each earth station, satellite and user terminal emit an RF signal with sufficient power to ensure that the intended receiver receives the RF signal with a desired quality of service. The quality of service of a communications link is dependent on the signal-to-noise ratio (SNR) of the RF signal. Different types of user terminals (portable, fixed, special, geographically specific, etc.) have associated minimum SNR levels required to afford a desired quality of service. Thus, each satellite must transmit RF signals in associated subbands at varying power levels to maintain the desired quality of service which depend upon the intended user terminal type.
In addition, satellites vary the RF signal transmission power levels between subbands and between channels in a subband to account for system factors, such as the position of the beam spot for an associated subband, the number of user terminals assigned to the subband, the position of the user terminals within the associated beam spot, the amount of interference between the user terminal and satellite (rain, fog, clouds, etc.), the distance to the user terminal and the like. The above-noted system factors continuously change, and thus the satellite must continuously update the transmission power level of RF signals in each subband to each user terminal.
However, each satellite is afforded a limited supply of power. Each satellite has many power demands upon this limited supply. Thus, it is desirable to maximize the transmission efficiency. To do so, satellite antennas have been implemented with nonlinear amplifiers which drive the antenna array to transmit the RF signals. However, driving the nonlinear amplifiers too far into saturation will cause excessive intermodulation distortion as well as reduced amplifier reliability.
A need remains for a satellite system which optimizes the satellite transmitter operating power level, while maintaining a desired quality of service at each user terminal.
Moreover, proposed satellite systems have been unable satisfactorily control the "effective isotropic radiated power" (EIRP) emitted by an earth station and received by a corresponding satellite. As noted above, an earth station passes RF signals to a desired user terminal along a forward link of a communication channel. In the forward link, the associated satellite receives each RF signal via a feeder link with the earth station. The satellite then retransmits this received RF signal in the subband of the beam spot containing the destination user terminal. The satellite must transmit the RF signal at a power level sufficient to provide the desired signal-to-noise ratio (SNR) and quality of service at the user terminal. A need remains for a satellite system which affords control at the earth station of the power output of the satellite for each channel.
Each satellite may receive RF signals along multiple feeder links from multiple earth stations. Each earth station is located a different distance from the satellite and at a different point within the satellite field of view. Consequently, RF signals from different earth stations may be received at different power levels. Power fluctuations in the received RF signal may further vary due to signal interference, such as clouds, rain and the like. Hence, RF signals from an earth station covered by clouds would be weaker than an RF signal from an earth station with no cloud cover. A need remains for an improved feeder link between the earth stations and satellites.
The present invention provides an improved power control satellite subsystem which overcomes the disadvantages discussed above and experienced in the past.